Recently, for the purpose of preventing dirt deposition and occurrence of flaws in the production processes of various members and/or in the shipping and transportation processes of various members, protection films or protection sheets in each of which an adhesive agent layer is laminated on the base made of a synthetic resin have been widely used. Among such films and sheets, films or sheets to protect the members for outdoor applications are in particular required to have antiweatherability and light resistance. Protection films or protection sheets have to be peeled off after completion of their role, are required to strongly adhere to adherends while being required, and on the other hand, are required to be easily peeled off when the adherends are used. In particular, in optical members such as displays, laminates in each of which a protection film or a protection sheet is bonded are disposed in a high temperature atmosphere or subjected to a cleaning process, and hence the protection film or the protection sheet are required to have high adhesiveness and at the same time, to be able to be peeled off with light force. As methods for obtaining an adhesive agent excellent in repeelability, the following methods are known.
There is known a method in which on a copolymer obtained from a monomer mixture prepared by adding to 100 parts by weight of a (meth)acrylic acid dialkyl ester (a) mainly composed of a (meth)acrylic acid alkyl ester with the alkyl group having 8 to 10 carbon atoms, 1 to 15 parts by weight of a carboxyl group-containing copolymerizable compound (b) and 3 to 100 parts by weight of a vinyl ester (c) of an aliphatic carboxylic acid having 1 to 5 carbon atoms, an adhesive agent layer prepared by mixing a cross-linking agent therein equivalent or more relative to the carboxyl group of the component (b) is disposed, and the adhesive agent layer is subjected to a cross-linking treatment to set the gel fraction of the adhesive agent layer at 60% or more (see, for example, Patent Literature 1).
There is known a method in which a cross-linkable composition is subjected to a cross-linking reaction so as to have a gel fraction of 80% or more, the cross-linkable composition including:
the component (A): a (meth)acrylic polymer having a glass transition point temperature of −40° C. or lower, prepared by copolymerizing at least a (meth)acrylic acid alkyl ester and a functional group-containing monomer;
the component (B): a (meth)acrylic polymer mainly composed of a (meth)acrylic acid ester having a glass transition temperature of 80° C. or higher; and
the component (C): a cross-linking agent, wherein 5 to 20 parts by weight of the component (B) is included in relation to 100 parts by weight of the component (A) (see, for example, Patent Literature 2).
There is known a method in which the following are mixed:
(A) 100 parts by weight of a (meth)acrylic polymer prepared by copolymerizing at least the following components (a1) and (a2):
(a1) a (meth)acrylic acid alkyl ester, and
(a2) a (meth)acrylic monomer having a hydroxyl group or a carboxyl group,
(B) 0.1 to 20 parts by weight of dibutyl sebacate; and
(C) 1 to 10 parts by weight of an isocyanate-based cross-linking agent and/or 1 to 10 parts by weight of an aziridine-based cross-linking agent and/or 0.01 to 5 parts by weight of an epoxy-based cross-linking agent (see, for example, Patent Literature 3).
There is known a method in which in an acrylic resin (A) prepared by copolymerizing a (meth)acrylic acid alkyl ester (a1) and a carboxyl group-containing unsaturated monomer (a2), a cross-linking agent (B) is mixed in an amount of 0.5 mol or less in relation to 1 mol of the carboxyl group in the acrylic resin (A) (see, for example, Patent Literature 4).
There is known a method in which a resin composition the resin component of which is composed only of an acrylic resin (A) substantially having no ethylenically unsaturated group is irradiated with an active energy ray (see, for example, Patent Literature 5).
With respect to an adhesive agent including an acrylic acid ester-based resin having a weight average molecular weight of 1000000 to 2000000 as measured by the GPC method relative to polystyrene standards and an adhesive agent, when the adhesive agent is immersed in ethyl acetate at 23° C. for 24 hours, and then taken out from the ethyl acetate and dried at 110° C. for 1 hour, the gel fraction (Y) represented by the following formula is made to be 0 to 30% by weight (see, for example, Patent Literature 6):Y=100×W2/W1 (in the formula, W1 is the weight of the adhesive agent before drying and W2 is the weight of the adhesive agent after drying)
There is known a method in which in 100 parts by weight of an acrylic resin mainly composed of a structural unit derived from a specific acrylic acid ester, 0.2 to 8 parts by weight of an ionic compound having an organic cation and being solid at room temperature is mixed (see, for example, Patent Literature 7).
Recently, attention has also been paid to moth-eye structure capable of obtaining super antireflection effect without using a conventional optical interference film, as a technique to reduce the surface reflection of display devices. The moth-eye structure allows the refractive index variation in the boundary between the external environment (air) and the surface of a product to be quasi-continuous by arranging, on the surface of the product to be subjected to an antireflection treatment, an asperity pattern further finer than the asperity pattern formed in an anti-glare (AG) film, having the asperity spacings equal to or smaller than visible light wavelength without spaces between the asperities; thus, light is almost completely allowed to pass through the refractive index interface without being affected by the refractive index interface, and thus, the optical reflection on the surface of the product can be made to almost disappear (see, for example, Patent Literature 8 and Patent Literature 9).